Method for Seamless Inter-Frequency Hard Handover in Radio Communication System

ABSTRACT

A method for seamless inter-frequency hard handover in a radio communication system is disclosed. The method for seamless inter-frequency hard handover includes the steps of: a) at a mobile station, blocking a first uplink carrier frequency used for communication, transmitting a direct sequence spread preamble signal through a second uplink carrier frequency for a short time, and continuously performing the communication through the first uplink carrier frequency; and b) at a target base transceiver station, acquiring an uplink synchronization of a mobile station based on the preamble before performing handover.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/466,053 filed on Jul. 9, 2003, pending, which is a non-provisionalapplication of International Application No. PCT/KR01/02314, filed Dec.31, 2001.

TECHNICAL FIELD

The present invention relates to a handover method in a radiocommunication system; and, more particularly, to a method capable ofimplementing a seamless inter-frequency hard handover in a directsequence code division multiple access (DS-CDMA) system and a computerreadable recording medium for executing the method.

BACKGROUND ART

It is widely known that a soft handover, in which a frequency changedoes not occur, can be used generally for handover between base stationsof one wireless communication service provider in a CDMA cellular systemsince all base stations of the one wireless communication serviceprovider can use a same frequency for the soft handover.

The soft handover is a method for maintaining a communication link bysimultaneously transceiving communication signal with both a source basestation and a neighbor base station without changing a communicationfrequency when a mobile station is located at a cell boundary of the twobase stations, i.e., when the mobile station moves from a coverage ofthe source base station to a coverage of the neighbor base station, andthen disconnecting the communication link with the source base station,if a signal intensity of the source base station is weaken below astandard signal intensity as maintaining continuously the communicationlink with the neighbor base station. The above-mentioned soft handoverprovides the unstrained handover by eliminating aninstant-disconnection, which is a chronic problem of an analog system,decreases a probability of call loss and maintains high qualitycommunication.

However, the soft handover is not applicable in case a certain wirelesscommunication service provider allocates different number of frequenciesto the neighbor base stations according to a call density by consideringan economy of a network design and an efficiency of investment, i.e.,the neighbor base stations use different frequencies. That is, if themobile station using a specific frequency of the source base station ismoving to the cell of the neighbor base station, which does not equipthe specific frequency, the soft handover cannot be applicable.Furthermore, the soft handover cannot be applicable between basestations possessed by two wireless communication providers usingdifferent frequencies although they adopt the same CDMA scheme. Forthese cases, the hardware handover has to be used.

The hard handover needs to be performed between frequencies in case ofthe handover from a wideband-CDMA (W-CDMA) time division duplex (TDD) toa W-CDMA frequency division duplex (FDD) or handover from global systemfor mobile communications (GSM) to the W-CDMA FDD.

Referring to FIG. 1, an example of the hard handover between a sourcebase station 2 and a target base station 3 is explained as follows.

As shown in FIG. 1, the target base station 3 does not supportcommunication frequencies f₁ and f₁₁ of a current mobile station 1.

In FIG. 1, in case of the mobile station 1 having a dual-mode receiver,the mobile station 1 can measure a signal intensity of a new frequencyf₂ while demodulating downlink signals through the currently establishedfrequency f₁ and acquire synchronization of signals transmitted from thetarget base station 3.

Such the dual-mode receiver needs an additional hardware for radiofrequency (RF) compared to a single-mode receiver and thus thecomplexity of a mobile device is increased.

To overcome above-mentioned problems, a compressed mode is defined in anasynchronous W-CDMA (FDD) standard (Release '99) of 3rd generationpartnership project (3GPP), which was released at September 2000.

FIG. 2 illustrates an example of compressed mode transmission.

In the 3GPP standard, a frame has a length of 10 msec and consists of 15slots.

For a transmission gap (TG) region 7 in a compressed frame, datatransmission is not permitted. Instead of permitting the datatransmission, a rate of frame errors of the compressed frame ismaintained identical to that of a normal frame 5 by keeping atransmitting power at a slot region 6 in the compressed frame higherthan a power of the normal frame 5.

The mobile station 1 having the single-mode receiver can search thesignal intensity of the new frequency f₂ in downlink on the handoversituation shown in FIG. 1 by using the compressed mode of FIG. 2. Thatis, it is possible to search the signal intensity by dropping thecurrent established communication frequency f₁, changing to a frequencyf₂ and measuring the signal intensity of f₂ in the TG region and afterthe TG region is over, demodulating the call channel of the frequencyf₁.

In the 3GPP (FDD) standard, the compressed mode is defined at not onlythe downlink but also the uplink. The downlink and uplink can beoperated simultaneously as the compressed mode and only one of thedownlink and the uplink can be operated as the compressed mode. A reasonof defining the compressed mode in the uplink is for prevention ofinterference to the downlink when the mobile station 1 measures afrequency of the uplink and the downlink of a neighbor system such as3GPP TDD or GSM. Therefore, even though the mobile station 1 employs thedual-mode receiver, the uplink needs to be operated as the compressedmode in case that the mobile station 1 measures the downlink of othersystem using a frequency similar to the frequency of the uplink.

In shortly, it is possible that the mobile station 1, which satisfiesthe 3GPP (FDD) standard, monitors a new frequency f₂ of the downlinkbefore disconnecting the current established call channel completely inthe handover situation in FIG. 1 and call disconnection of the downlinkcan be avoided although there occurs the hard handover to the newfrequency f₂ since the synchronization of the downlink transmitted fromthe target base station 3 can be acquired by using a synchronizationchannel of f₂ and a common pilot channel.

On the other hand, in case of the uplink, since the target base stationreceives no signal before the mobile station 1 drops the currentestablished frequency f₁′ and transmits signals by using a new frequencyf₂′, i.e., the hard handover occurs, the synchronization of the uplinkneeds to be started at the target base station 3 from a moment that thehard handover occurs. There occurs call disconnection since at least oneframe is required to acquire the synchronization of the uplink even ifan outperformed searcher is used in the target base station 3.

Moreover, since, according to the 3GPP W-CDMA (FDD) scheme,corresponding base stations operate in asynchronization, the target basestation 3 cannot detect a round trip delay between the mobile station 1and the target base station 3 and therefore, a time for acquiringsynchronization in the target base station 3 may be more than severalframes since a search window size becomes very large, which a searcherhas to search, in case that a coverage area of the base station is huge.In this case, several frame disconnection may happen and current calldisconnection also may be happened in more serious case. Also, in thiscase, a power may not be controlled properly, so that a capacity of theuplink of the target base station 3 may be incredibly decreased.

In the 3 GPP W-CDMA standard (Release '99), it is possible to performthe handover only in case a difference between a system frame number(SFN) of the target base station 3 and a connection frame number (CFN)of the mobile station 1 is known to the network. Therefore, the mobilestation 1 needs to detect the SFN information of the target base station3 by demodulating a common channel of the downlink of the target basestation 3 before performing the handover and transmit the SFNinformation and a frame offset, which is the difference between the CFNof the mobile station 1, to the base station controller 4. Therebyallowing the base station controller 4 to decide an exact handover time,resulting in performing the handover. Above-mentioned operations arewell performed in the soft handover between same frequencies. However,in case of the hard handover between different frequencies, the mobilestation 1 should use the compressed mode of the downlink for acquiringthe SFN information of the target base station 3.

However, in the standard (Release '99), it is impossible to acquire theSFN information by using the compressed mode since at least 50 mseccontinuous demodulating time is required in the downlink for acquiringthe SFN information. In case of the hard handover, since the mobilestation 1 has to acquire the SFN information after being completelydisconnected with the current established frequency and being connectedto a new frequency, there may occur at least S0 msec additional calldisconnection.

The above-mentioned problems are not limited to the inter-frequency hardhandover in the W-CDMA FDD and they may happen when a multimode devicehaving the dual-mode receiver or the single-mode receiver such asGSM/WCDMA FDD multimode device or W-CDMA TDD/W-CDMA FDD multimode deviceperforms the hard handover from the GSM system to the W-CDMA FDD systemor from the W-CDMA TDD system to the W-CDMA FDD system.

As mentioned above, the disconnection is inevitable for performing theinter-frequency hard handover defined in the 3GPP W-CDMA FDD standard.Specially, in case of the mobile station having the single modereceiver, since the compressed mode is used in the downlink forsearching signals of the target base station so the frame offset betweenthe target base station and the mobile station is not known to thenetwork. In this case, the disconnection problem becomes more serioussince at least 50 msec disconnection is generated during performing theinter-frequency handover. This is indicated as a problem in the 3GPP.Therefore, a handover method, which performs the inter-frequency hardhandover without disconnection, is required for addressing the problemsin the asynchronous W-CDMA standard.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodcapable of implementing a seamless inter-frequency hard handover in awireless communication system such as a DS-CDMA system and a computerreadable recording medium storing instructions for executing the method.

In accordance with an aspect of the present invention, there is provideda method for performing seamless inter-frequency hard handover in aradio communication system, including the steps of: a) a mobile station,disconnecting a first uplink carrier frequency used for communication,transmitting a direct sequence spread preamble signal through a seconduplink carrier frequency for a short time, and continuously performingthe communication through the first uplink carrier frequency; b) atarget base station, acquiring an uplink synchronization of the mobilestation by using the direct sequence spread preamble; and c) performingthe hard handover by using the uplink synchronization.

In accordance with another aspect of the present invention, there isalso provided a method for performing a seamless inter-frequency hardhandover in a radio communication system in case that a base stationcontroller (or a radio network) dose not know a frame offset, which is adifference between a connection frame number (CFN) of a mobile stationand a system frame number (SFN) of a target base station, including thesteps of: a) the mobile station, completely disconnecting a first uplinkcarrier frequency used for communication, transmitting a direct sequencespread preamble (or pilot) through a second uplink carrier frequency fora short time, and continuously performing the communication through thefirst uplink carrier frequency; b) the target base station, acquiring anuplink synchronization of the mobile station by using the preamblebefore performing the hard handover; c) the target base station, afteracquiring the uplink synchronization, transmitting a direct sequencespread AI as a response for the acquisition of the uplinksynchronization for a short time through a new downlink frequency; d)the mobile station, detecting the acquisition indicator (AI); e) thebase station controller, calculating a frame offset by using the SFN,which is used for transmitting the AI and the CFN, which is used forreceiving the AI, and transmitting the calculated frame offset to thetarget base station; and f) the base station controller, instructing themobile station and the target base station to perform the handover.

The present invention transmits a preamble (or a pilot) through newfrequency f₂′ in a transmission gap (TG) by using an uplink compressedmode before a mobile station completely disconnects a currentlyestablished communication in an inter-frequency hard handover situationdescribed in FIG. 1.

The present invention provides a method for seamless inter-frequencyhard handover by acquainting a synchronization of an uplink by using apreamble (or pilot) transmitted from a target base station before acurrently established communication is completely disconnected.

In the present invention, the target base station transmits anacquisition indicator (AI) through downlink for a fast response ofacquisition of the preamble (or pilot) transmitted from a mobile stationin the transmission gap (TG).

The present invention also prevents an additional call disconnection incase a base station controller does not know a frame offset, which is adifference between SFN of the target base station and CFN of the mobilestation, by providing a method that a network knows the frame offsetbefore performing the hard handover.

In a difference way of a convention inter-frequency hard handover havingproblem of at least more than one frame call disconnection, the presentinvention provides a method for seamless inter-frequency hard handoverby transmitting a preamble (or pilot) through new frequency with acompressed mode, acquainting a synchronization of an uplink by using apreamble (or pilot) transmitted from a target base station before acurrently established communication is completely disconnected.

The present invention also performs the hard handover quickly in anetwork by the target base station transmits the acquisition indicator(AI) according to a received preamble (or pilot) transmitted from amobile station in a transmission gap (TG).

In a difference way of a convention inter-frequency hard handover havingproblem of at least more than 50 msec call disconnection in case that abase station controller dose not know a frame offset, which is adifference between SFN of a target base station and CFN of a mobilestation, the present invention provides a method for seamlessinter-frequency hard handover by providing a method that a network knowsthe frame offset before performing the hard handover.

The present invention can be implemented to not only an inter-frequencyhard handover in W-CDMA FDD system but also a hard handover to W-CDMAFDD or from GSM to W-CDMA FDD in W-CDMA TDD system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a view illustrating a typical wireless communication systemfor performing an inter-frequency hard handover;

FIG. 2 is a diagram showing a structure of a typical compressed modetransmission;

FIG. 3 is a diagram depicting an example of an uplink transmission,which transmits a preamble through a new frequency by using a compressedmode of a single frame configuration for a seamless inter-frequency hardhandover in accordance with an embodiment of the present invention;

FIG. 4 is a diagram providing an example of an uplink transmission,which transmits a preamble through a new frequency by using a compressedmode of a double frame configuration for a seamless inter-frequency hardhandover in accordance with an embodiment of the present invention;

FIG. 5 is a diagram representing an example of uplink and downlinktransmission in case of a mobile station having a dual-mode receiver fora seamless inter-frequency hard handover in accordance with anembodiment of the present invention;

FIG. 6 is a diagram illustrating an example of uplink and downlinktransmission in case of a mobile station having single-mode receiver fora seamless inter-frequency hard handover in accordance with anembodiment of the present invention;

FIG. 7 shows a signal flow between systems in case a target base stationdoes not transmit an AI through a downlink when a network knows a frameoffset for a seamless inter-frequency hard handover in accordance withthe present invention;

FIG. 8 is a diagram illustrating a compressed mode pattern of the 3GPPW-CDMA employed in the present invention;

FIG. 9 exemplifies a timing chart showing that a target base stationdecides a searching region for a preamble transmitted from a mobilestation having a dual-mode receiver in case a frame offset is known to anetwork for a seamless inter-frequency hard handover in accordance withthe present invention;

FIG. 10 presents a signal flow between systems in case a target basestation transmits AI through a downlink when a frame offset is known toa network for a seamless inter-frequency handover in accordance with thepresent invention;

FIG. 11 is a timing chart in case a target base station transmits an AIfor detecting a preamble when a frame offset is known to a network for aseamless inter-frequency handover in accordance with the presentinvention;

FIG. 12 illustrates a signal flow chart in case one class of AI and oneclass of preamble is transmitted when a frame offset is not known to anetwork for a seamless inter-frequency handover in accordance with thepresent invention;

FIG. 13 is a timing chart illustrating that a target base stationsearches a preamble in a corresponding region of each frame from amoment it receives a preamble search order in case a frame offset is notknown to a network for a seamless inter-frequency handover in accordancewith the present invention;

FIG. 14 is a view illustrating that a base station controller calculatesa frame offset by using an SFN from a target base station and a CFN froma mobile station in case the frame offset is not known to a network fora seamless inter-frequency handover in accordance with an embodiment ofthe present invention;

FIG. 15 is a view describing that a base station controller calculates aframe offset by using an SFN of a target base station and a CFN of amobile station in case the frame offset is not known to a network for aseamless inter-frequency handover in accordance with another embodimentof the present invention;

FIG. 16 is a signal flow chart in case two classes of AI and two classesof preamble are transmitted when a frame offset is not known to anetwork for a seamless inter-frequency handover in accordance with thepresent invention;

FIG. 17 shows an example of operations of a mobile station and a targetbase station in case two classes of AI and two classes of preamble aretransmitted when a frame offset is not known to a network for a seamlessinter-frequency handover in accordance with the present invention; and

FIG. 18 provides a flowchart illustrating operations of a mobile stationand a base station using the method of FIG. 16 for a seamlessinter-frequency handover in accordance with the present invention.

MODES FOR CARRYING OUT THE INVENTION

With reference to the accompanying drawings, the preferred embodimentsof the present invention will be described in detail hereinafter.

FIG. 1 shows an example of performing a hard handover between two basestations, i.e., a source base station 2 and a target base station 3. Asshown in FIG. 1, the target base station 3 does not supportcommunication frequencies f₁ and f₁′ of a mobile station 1.

FIGS. 3 and 4 exemplify preamble transmission methods of an uplink,which use an uplink compression mode of the present invention forimplementing a seamless inter-frequency hard handover in a samesituation is in FIG. 1.

FIG. 3 shows a single frame compression mode and FIG. 4 represents adouble frame compression mode.

In the present invention, the mobile station 1 transmits a preamble 8through a new frequency f₂′ in a TG region by using the uplinkcompressed mode before disconnecting the current established frequencyf₁′ completely as shown in FIGS. 3 and 4. At this moment, the preamble 8transmitted from the mobile station 1 is a direct sequence bandwidthspread signal and a code sequence used for bandwidth spread needs to beknown to the target base station 3 before the preamble is transmitted.

For seamless inter-frequency hard handover, before performing the hardhandover, the target base station 3 acquires the synchronization ofsignals transmitted from the mobile station 1 by using the preamble 8 ofthe uplink at which the mobile station 1 transmits signals through thenew frequency f₂′ by using the uplink compressed mode.

In the present invention, the mobile station 1 has “Guard Time” before 9or after 10 transmitting the preamble 8 in the uplink compressed mode,and the target base station 3 and a base station controller 4, i.e., anetwork should know the “Guard Time” (T_(grd)) 9 before the preamble 8transmitted. This has to be newly defined in the 3GPP standard.

In the present invention, after the target base station 3 acquires thesynchronization of signal transmitted from the mobile station 1 by usingthe preamble 8 of the uplink transmitted through the frequency f₂′, thetarget base station 3 can transmit an acknowledgment (ACK) of acquiringthe synchronization (through a wired network) to the base stationcontroller 4 or through the downlink of the frequency f₂ to the mobilestation 1.

FIGS. 5 and 6 show an example that the target base station 3 transmitsan acknowledgment for the synchronization in the form of an acquisitionindicator (AI) to the mobile station 1 by using the downlink offrequency f₂ after acquiring the synchronization of signals transmittedfrom the mobile station 1 by using the preamble 8 of the uplinktransmitted through the frequency f₂′.

FIG. 5 shows an example of data transmitting between a mobile stationand a base station in case of the mobile station having a dual-modereceiver and FIG. 6 shows an example of data transmitting between amobile station and a base station in case of the mobile station having asingle-mode receiver.

As illustrated in FIGS. 5 and 6, the AI transmitted by the target basestation 3 is not related with a structure of the receiver of the mobilestation, whereas a structure of frames transmitted from the source basestation 2 and a role of the frames at receiver of the mobile station aredepending on the structure of the receiver of the mobile station. Thatis, in case the mobile station 1 has the single-mode receiver, thesource base station 2 transmits the compressed frame in the downlink asshown in FIG. 6 as a frame, which is corresponding to a location of theAI 11 transmitted from the target base station 3, on the other hand,case of the mobile station 1 has the dual-mode receiver, a normal frameis transmitted as shown in FIG. 5.

As shown in FIGS. 5 and 6, the target base station 3 transmits the AIafter a certain period time is passed by considering the processing timeafter receiving the preamble 8 from the mobile station 1 and the mobilestation 1 has to know a start point of transmitting the AI.

When demodulating the AI, the mobile station with the dual-mode receiveror the single-mode receiver can perform a non-coherent demodulation orcan perform a coherent demodulation by using a common pilot channeltransmitted through the downlink of the frequency f₂ from the targetbase station 3. Although it is not shown in the drawings, the targetbase station 3 transmits other channel of the downlink of the frequencyf₂ such as a common pilot channel, a synchronization channel and datachannel for other mobile station 1 and so on in a code division scheme.

In case the mobile station 1 has the single-mode receiver, the mobilestation 1 uses the common pilot channel of the target base station 3,which is received through the frequency f₂ in the TG region 7, tocoherently demodulate the AI.

In case the mobile station 1 has the single-mode receiver 1, the “GuardTime” 13 has to exist between the end of the AI transmission and the endof the TG region.

In the uplink preamble transmission method of the present inventionshown in FIGS. 3 to 6, the mobile station 1 can transmit a same preambleseveral times by repeating the compressed mode several times so as toincrease the probability of the target base station acquiring thesynchronization. At this moment, the compressed mode pattern istransmitted from the base station controller 4 to the mobile station 1and the target base station 3. Also, in the AI transmission of thedownlink described in FIGS. 5 and 6, the target base station 3 cantransmit the AI several times to increase the probability of the mobilestation 1 detecting the AI. Parameters related to the compressed modepattern are defined in the 3GPP TS25. 525 standard.

FIG. 7 is a diagram illustrating a signaling procedure between themobile station 1, the source base station 2, the target base station 3and the base station controller 4 during performing the hard handover incase that the frame offset is known to the base station controller 4 andthe target base station 3 does not transmit the acquisition indication(AI) for the acquisition of the downlink preamble to the mobile station1, wherein the frame offset is a difference between the SFN of thetarget base station 3 and the CFN of the mobile station 1.

In FIG. 7, it is assumed that the frame offset is known to the basestation controller 4 but the target base station 3 does not transmit theAI to the downlink. Referring to FIG. 7, at time0 14, the mobile station1 and the source base station 2 are communicating through the frequencyf₁ (f₁′); the target base station 3 has resources to support the mobilestation 1 through the frequency f₂ (f₂′); the frame offset and a chipoffset were known to the base station controller 4 which OVSF_(target)from the target base station 3, the mobile station acquired thesynchronization of the downlink of the target base station 3; and thetarget base station 3 does not acquire the synchronization of theuplink.

That is, at the time0 14, the mobile station 1 and source base station 2are communicating through f₁ (downlink) and f₁′ (uplink); the targetbase station 3 has reported to the base station controller 4 that thereare resources at the f₂ (f₂′) link to support the mobile station 1 andalso the target base station 3 has reported an orthogonal variablespreading factor (OVSF) code of the downlink to be used by the mobilestation 1 at the downlink to the base station controller 4 afterperforming the handover.

As mentioned above, the base station controller 4 already knows theframe offset and the chip offset at the time0 14. Herein, the frameoffset represents the difference between the SFN of the target basestation 3 and the CFN of the mobile station 1 and it is defined in thestandard TS25. 402 (Release '99) of 3GPP. Also, the SFN is a framenumber of a common control channel of the downlink and has a range of 0to 4095 and the CFN is a transport channel frame number, having a rangeof 0 to 255 and is determined after a communication link between themobile station 1 and a base stations is established.

The base station controller 4 can know the frame offset by receiving theframe offset from the mobile station before the time0 14 in case themobile station 1 has a dual-mode receiver or analogizing the frameoffset from information reported from other base stations. The chipoffset is a difference between frame boundary of a transport channel ofthe mobile station 1 and a frame boundary of a common control channel ofthe target base station 3 and has a range of 0 to 38399 chips. Themobile station 1 measures the chip offset by using the dual-modereceiver or using the compressed mode in case of a single mode receiverand the measured chip offset transmitted to the base station controller4. The chip offset is defined in TS25. 402 (Release '99).

Also, at the time0 14, the mobile station 1 has acquired thesynchronization of the downlink of the target base station 3 while thetarget base station 3 has not acquired the synchronization of theuplink.

The base station controller 4, which knows the frame offset, transmitsin step 15 the transport channel frame number (TGCFN) of starting thecompressed mode in the uplink of the mobile station 1 after the time014, information for a starting slot number of the TG (TGSN), informationfor the compressed mode pattern (TGL1, TGL2, TGD, TGPL1, TGPL2) andinformation for a total length of the compressed-mode TGPRC to themobile station 1 before TGCFN.

FIG. 8 shows parameters for the compressed mode pattern defined in the3GPP TS25.215. The mobile station 1 transmits the uplink preamble at atime decided by the compressed mode pattern received from the basestation controller 4. The mobile station 1 uses an open loop powercontrol when transmitting the first preamble. That is, the mobilestation 1 determines a transmission power of the first preamble by usingthe intensity of received signals measured at the downlink f₂ of thetarget base station 3 before the time0 14 or a signal to noise rate(Ec/Io) of received signals at the common pilot channel.

Also, the controller of base station 4, which knows the frame offset,transmits a transport channel frame number (TGCFN) of starting uplinkcompressed mode of the mobile station 1 after the time0 14, informationfor a starting slot number of each TG (TGSN), information of thecompressed mode pattern (TGL1, TGL2, TGD, TGPL1, TGPL2), information oftotal length of the compressed mode (TGPRC), a frame offset, informationfor a chip offset and a scrambling code number (SCID) to the target basestation 3 before the TGCFN 16 in step 16.

After then, the target base station 3 acquires the synchronization ofthe preamble transmitted from the mobile station 1 by using the frameoffset, the chip offset information, the SCID and the compressed modepattern received from the base station controller 4.

FIG. 9 provides a timing chart illustrating that the target base station3 decides a searching region for the preamble transmitted from themobile station 1 by using the information from the base stationcontroller 4.

Referring to FIG. 9, it is assumed that the mobile station 1 has thedual-mode receiver.

At first, the target base station 3 calculates the SFN corresponding tothe TGCFN by using the frame offset and a following equation EQ. 1.SFN mod 256=(frame offset+TGCFN)mod 256  EQ. 1

Since in the equation EQ. 1, the range of the SFN is 4096 and that ofthe TGCFN is 256, there are 16 SFNs satisfying the equation EQ. 1. Forexample, if the frame offset is 67 as shown in FIG. 9, the SFNs become123, 379, 635, . . . .

A base station searcher selects an SFN whose value is closest to a framecorresponding to a time of receiving a preamble searching order from thebase station controller 4 among the 16 possible SFNs as the SFNcorresponding to the TGCFN. Therefore, in FIG. 9, “123” is selected asthe SFN corresponding to the TGCFN since the “123” is closest to a time“120”, which is a time of receiving the preamble searching order fromthe base station controller 4. The target base station 3 searches theuplink preamble by setting up a searching region from a time 17, whichis β chips away from boundary of a corresponding frame (e.g., 123 framein FIG. 9), to a time which is “preamble length+2τ_(max)”. Herein, β isdefined as follows:β=chip offset+TGSN×2560+T ₀ +T _(grd)  EQ. 2

In the equation EQ. 2, T₀ is a difference between a downlink time (DLDPCH_(norm)) and an uplink time of the mobile station 1 and it isdefined as 1024 chips in the 3GPP standard. As before mentioned T_(grd)is a Guard Time before the mobile station 1 transmits the preamble andthe T_(grd) should be known to the target base station 3. β may belarger or smaller than 38400 chips.

In FIG. 9, 2τ_(max) is the maximum round trip delay corresponding to acell coverage of the target base station 3 and equal to a search windowsize of the target base station 3. If it is assumed that the chiptransmission speed is 3.84 Mcps and the cell coverage is 20 km, 2τ_(max)becomes almost 512 chips.

In case the preamble searching is failed, the target base station 3searches the uplink preamble by setting up the search region from a time18, which is β chips away from a boundary of a next frame (e.g., 129frame in FIG. 9) designated by a compressed mode pattern received fromthe controller of base station 4, to a time, which is “preamblelength+2τ_(max)”. The target base station 3 repeatedly performs theabove-mentioned procedure until the preamble is detected.

The target base station 3 acquires the uplink synchronization by usingthe configuration of FIG. 9 and reports it to the base stationcontroller 4 in step 19. At this moment, the target base station 3 alsotransmits the receiving intensity of the preamble (e.g., E_(c)/I_(o)value) transmitted from the mobile station 1. Then, in step 20, the basestation controller 4 instructs the source base station 2, the mobilestation 1 and the target base station 3 to perform the handover. At thistime, the base station controller 4 transmits a CFN at a time at whichthe handover starts to the source base station 2, the mobile station 1and the target base station 3 and further transmits OVSF_(target) to beused as a channel spread code of a new downlink in the handover to themobile station 1.

The mobile station 1, which received the handover instruction from thebase station controller 4, stops transmitting the preamble using thecompressed mode. Also, the mobile station 1 disconnects the call withthe source base station 2 at the CFN received from the base stationcontroller 4 and starts to communicate with the target base station 3 byusing a new frequency f₂ (f₂′) in step 21. Before starting to thecommunicate between the target base station 3 and the mobile station 1at the corresponding CFN, the uplink and the downlink have been alreadysynchronized by the method of the present invention and, therefore, acall disconnection there between does not occur.

FIG. 10 is a diagram illustrating a signaling procedure between themobile station 1, the source base station 2, the target base station 3and a base station controller 4 during performing the hard handover incase that the frame offset is known to the base station controller 4 andthe target base station 3 transmits the AI for the synchronizationacquisition for the uplink to the mobile station 1.

In FIG. 10, it is assumed that the base station controller 4 knows theframe offset and the target base station 3 transmits the AI through thedownlink. In FIG. 10, at a time0 14, the mobile station 1 and the sourcebase station 2 are communicating with each other through f₁ (f₁′), thetarget base station 3 has resources at the f₂ (f₂′) link to support thecurrent mobile station 1; the base station controller 4 knows the frameoffset and chip offset and has received OVSF_(target) from the targetbase station 3; the mobile station 1 has acquired already thesynchronization of the downlink of the target base station 3; and thetarget base station 3 does not acquire the synchronization of the uplinkyet.

The signaling procedure of FIG. 10 is similar to that of FIG. 7 but thebase station controller 4 transmits not only parameters related to acompressed mode pattern but also OVSF_(target) code information to beused for the downlink in the handover, when the base station controller4 instructs the mobile station 1 to transmit an uplink preamble in step22. Also, after acquiring the uplink synchronization, in step 23, thetarget base station 3 transmits the AI for the synchronization throughthe downlink by using a configuration shown in FIG. 11.

When transmitting the AI, the target base station 3 uses anOVSF_(target) code as a channel spread code, wherein the OVSF_(target)code is identical to the OVSF_(target) code sent when instructing themobile station 1 to transmit the uplink preamble in the step 22, and themobile station 1 uses the OVSF_(target) when demodulating the AI.

In a method shown in FIG. 10, in which the target base station 3transmits the AI in case the base station controller 4 knows the frameoffset, the procedure of searching the uplink preamble performed by thetarget base station 3 is identical to a method shown in FIG. 7 in whichthe target base station 3 down not transmit the AI.

FIG. 11 shows an example that the target base station 3 succeeds indetecting a preamble at a second time after failing to detect preambleat a first time, wherein the preamble is transmitted from the mobilestation, having a single-mode receiver.

The target base station 3 transmits the AI after succeeding in detectingthe uplink preamble, and the AI is transmitted at a second point 24,which is γ chips away from a first point, β chips away from a boundaryof a frame designated by the compressed mode pattern, wherein thedesignated frame is a 129 frame in FIG. 11. In here, β is defined in theequation EQ. 2 and γ should satisfy a following equation EQ. 3. The γhas to be known to the target base station 3 and the mobile station 1and, therefore, this should be newly defined in the 3GPP standard.γ>2τ_(max) +T _(pre)  EQ. 3

The mobile station 1 demodulates the AI at times, at which the mobilestation is expected to receive the AI, such as 25 and 26 in FIG. 11,wherein the mobile station 1 has known β and γ and acquired a frameboundary 27 of signals received from the target base station 3. In casethe mobile station has the single-mode receiver, as like in FIG. 11, thecompressed mode of downlink is used. The mobile station 1, whichreceived the AI does not transmit the preamble any more. If it fails todetect the AI, the mobile station 1 re-transmits the preamble in a nextcompressed mode region. After transmitting the preamble, if the preambleis detected again in a next compressed mode, the target base station 3re-transmits the AI.

The target base station 3 detecting the preamble reports the detectionof the preamble to the base station controller 4 and then the basestation controller 4 instructs the source base station 2, the mobilestation 1 and the target base station 3 to perform the handover. At thismoment, the base station controller 4 transmits the CFN at a point ofthe handover started to the source base station 2, the mobile station 1and the target base station 3. The mobile station 1, which received thehandover instruction from the base station controller 4, disconnectscommunication with the source base station 2 at the CFN and starts tocommunicate with the target base station 3 by using the new frequency f₂(f₂′). Before starting to communicate with the target base station 3 andthe mobile station 1 at corresponding CFN, synchronization with theuplink and downlink is acquired by using the method of the presentinvention and, therefore, the call disconnection there between does notoccur.

In the W-CDMA FDD standard (Release '99) of the 3GPP, if the basestation controller 4 does not know the frame offset, which is adifference between the SFN of the target base station 3 and the CFN ofthe mobile station 1, when performing the handover, the mobile station 1disconnects the established frequency completely and re-acquires the SFNof the target base station 3 through a new frequency for performing thehard handover. Therefore, at least 50 msec of additional calldisconnection may occur between the mobile station 1 and the target basestation 3.

For the case that the base station controller 4 does not know the frameoffset, the handover method of the present invention forces the basestation controller to know the frame offset before performing thehandover by using the AI transmitted from the target base station 3 tothe mobile station 1. This will be explained in detail with reference toFIG. 12.

FIG. 12 shows a signaling procedure between the mobile station 1, thesource base station 2, the target base station 3 and the base stationcontroller 4 during performing the hard handover in case that the basestation controller 4 does not know the frame offset.

In FIG. 12, it is assumed that the base station controller 4 does notknow the frame offset. At time0 28 in FIG. 12, the mobile station 1 andthe source base station 2 are communicating with each other through thefrequency f1 (f1′); the target base station 3 has resources at the f2(f2′) link to support the mobile station 1; the base station controller4 knows the chip offset but does not know the frame offset, and hasreceived OVSF_(target) from the target base station 3; the mobilestation 1 has already acquired the synchronization of the downlink ofthe target base station 3; and the target base station 3 does notacquire the synchronization of the uplink yet.

That is, at time0 28, the mobile station 1 and the source base station 2are communicating with each other through the frequencies f₁ (downlink)and f₁′ (uplink); the target base station 3 has reported to the basestation controller 4 that there are resources at the f2 (f2′) link tosupport the mobile station 1; and an orthogonal variable spreadingfactor (OVSF) code of the downlink has been already reported to the basestation controller 4, wherein the OVSF code will be used in the downlinkby the mobile station 1 after the handover performed.

As mentioned above, the base station controller 4 knows the chip offsetbut does not know the frame offset at time0 28.

The signaling procedure shown in FIG. 12 is similar to that in FIG. 10.However, the base station controller 4 does not transmit the frameoffset information to the target base station 3 when instructing thetarget base station 3 to search an uplink preamble in step 30. It isbecause the base station controller 4 does not know the frame offsetinformation.

The target base station 3, which received the preamble searchinginstruction in the step 30, performs the preamble searching process fora corresponding region of the frame from the moment of receiving thepreamble searching order. This is illustrated in FIG. 13.

Since the target base station 3 does not know an SFN of a frame throughwhich the mobile station 1 transmits the preamble in step 31 but knowsinformation for a slot at which the preamble starts, the target basestation 3 sets up searching regions (38, 39, 40, . . . ) from a boundaryof each frame to a point, which is 2τ_(max) away from an offset ofβ_(mod) 38400 and searches the preamble. In here, “_(mod) 38400” is usedbecause, as mentioned, β may be larger than 38400.

The target base station 3, which succeeded in searching the preamble,transmits the AI in step 32 to the mobile station 1 through thedownlink, and, at the same time, reports the acquisition of the uplinksynchronization to the base station controller 4 in step 33. At thistime, the SFN corresponding to the AI is also transmitted. After then,the mobile station 1, which received the AI from the target base station3, transmits a CFN of the frame corresponding to the received AI to thebase station controller 4 in step 34. At this time, the base stationcontroller 4 calculates the frame offset in step 35 by using the SFNreceived from the target base station 3 and the CFN information receivedfrom the mobile station 1 for the AI transceiving time.

FIGS. 14 and 15 show a procedure of calculating the frame offsetperformed in the base station controller 4 by using the SFN informationreceived from the target base station 3 and the CFN information receivedfrom the mobile station 1.

After calculating (β+τ)_(mod) 38400, if the calculated value is smallerthan the chip offset, the base station controller 4 determines the valueof the frame offset as (SFN−CFN−1)mod 256 and, if otherwise, decides thevalue of the frame offset as (SFN−CFN)_(mod) 256. This is shown in anequation EQ. 4.frame offset=(SFN−CFN−1)_(mod) 256 for (β+τ)_(mod) 38400<chip offsetframe offset=(SFN−CFN)_(mod) 256 for (β+τ)_(mod) 38400·chip offset EQ. 4

When transmitting the AI, the target base station 3 uses the OVSF targetcode, which is identical to the OVSF target used when sending the uplinkpreamble transmitting instruction in the step 22 to the mobile station1, as a channel spread code and the mobile station 1 uses the sameOVSF_(target) when demodulating the AI.

After calculating the frame offset, the base station controller 4instructs the source base station 2, the mobile station 1 and the targetbase station 3 to perform the handover in steps 36 and 37. At this time,the base station controller 4 provides the mobile station 1 with the CFNin which the handover is performed and, in turn, the mobile station 1disconnects the current communication link with the source base station2 at the CFN and starts to communicate with the target base station 3through a new frequency f₂ (f₂′).

The base station controller 4 transmits the calculated frame offset andthe CFN at which the handover is performed to the target base station 3in step 37. At this time, the target base station 3 calculates an SFNcorresponding to the CFN at which the handover is performed by using theframe offset received from the base station controller 4 and receives anuplink DPCH at the same time transmitting, a downlink DPCH from themoment corresponding to the calculated SFN in step 38.

As mentioned above, although the base station controller 4 does not knowthe frame offset, the handover method of the present invention forcesthe base station controller 4 to know the frame offset before performingthe handover by using the AI transmitted from the target base station 3to the mobile station 1 and transmits the frame offset the target basestation 3 together with the handover instruction, so that, it ispossible to perform the seamless inter-frequency hard handover.

FIG. 16 shows a signaling procedure of a method using two classes ofpreambles and two classes of AIs in each of the mobile station 1 and thebase station to minimize false handover instruction given by the basestation controller 4 by minimizing the false detection probability ofthe mobile station 1 and the target base station 3 in case the basestation controller does not know the frame offset.

The mobile station 1, which received a preamble transmitting instructionfrom the base station controller 4 transmits the preamble₁ through thefrequency f₂′ by using the uplink compressed mode in step 41. At thistime, the target base station 3, which received the preamble searchinginstruction from the base station controller 4 searches the preamble₁according to the above-mentioned procedure described in FIG. 13. Aftersucceeding in searching the preamble, the target base station 3transmits an AI₁, which is an acknowledgement for succeeding insearching the preamble₁, the frequency f₂ in step 42. And then, themobile station 1 detects the AI₁, which is transmitted to the frequencyf₂₁ by using the downlink compressed mode or the dual-mode receiver and,in turn, transmits a preamble₂ to a frequency f₂′ at a next compressedframe in step 43.

After transmitting the preamble₂ the target base station 3 detects thepreamble₂ transmits an AI₂ in step 44, which is an acknowledgement forachieving to detect the preamble₂, to the mobile station 1, and reportsthe acquisition of the uplink synchronization to the base stationcontroller 4 at the same time of notifying an SFN through which the AI₂is transmitted to the base station controller 4 in step 45.

The mobile station 1 detects the AI₂ received through the frequency f₂by using the downlink compressed mode or the dual-mode receiver andreports the detection of AI₂ to the base station controller 4 at thesame time of notifying, CFN through which the AI₂ is also received tothe base station controller 4 in step 46. And then, the base stationcontroller 4 calculates the frame offset by using the equation EQ. 4with the SFN transmitted from the target base station 3 and the CFNtransmitted from the mobile station 1 and invokes the source basestation 2, the target base station 3 and the mobile station 1 to performthe handover in the steps 36 and 37. At this moment, the calculatedframe offset is transmitted to the target base station 3. Steps aftertransmitting the calculated frame offset are same as the steps in FIG.12.

FIG. 17 shows an example of an AI transmission of a base station and apreamble transmission of the mobile station 1 when the method in FIG. 16is used.

The mobile station 1 uses an open loop power P0 to transmit a preamble₁at the first time and in case of failing to receive the AI₁, increasesthe power up to P1 to re-transmit the preamble₁. In case of failing toreceive the AI₁, the above mentioned steps are performed repeatedly andin case of achieving to receive the AI₁, a preamble₂ is transmitted byusing a power (P2) identical to the power by which the preamble, isthereby transmitted.

When using the method described in FIG. 16, in case that the basestation controller 4 does not know the frame offset, a transmissioncycle of the preamble of the mobile station 1 has to be constant. “T” inFIG. 17 represents the transmission cycle of the preamble. That is, thebase station controller 4, which does not know the frame offset, shouldtransmit a compressed mode pattern satisfying by the above requirementto the mobile station 1 and the target base station 3.

FIG. 18 is a flowchart explaining operations of the mobile station andthe base station, which use the method of FIG. 16.

At first, the mobile station 1, which received a preamble transmissioninstruction from the base station controller 4 in step 29, transmits thepreamble₁ by using the uplink compressed mode in step 41 and then,detects the AI₁ in step 48. In the step 48, if the AI₁ is not detected,the mobile station 1 re-transmits the preamble₁ in the step 41, afterincreasing the power as much as A in step 47. On the other hand, if theAI₁ is detected, at a next compressed mode, the mobile station 1transmits the preamble₂ by using a power identical to the power by whichthe preamble₁ is finally transmitted in step 43.

After transmitting the preamble₂ if the detection of the AI₂ is achievedin step 49, the mobile station 1 transmits a CFN at which the AI₂ isreceived to the base station controller 4 in step of 46 and, if thedetection of the AI₂ is failed in the step of 49, the mobile station 1notices the failure of detecting the AI₂ to the base station controller4 in step 50.

Meanwhile, the target base station 3, which received a preamble searchinstruction from the base station controller 4 in step 30, searches thepreamble₁ in every search region in step 51 by using the procedure ofFIG. 13. The target base station 3 transmits the AI₁ in step 42 for thesearch region, at which the preamble₁ is detected in step 54, to themobile station 1. Therefore, the AI₁ may be transmitted more than onetime during the T in FIG. 17. In step 52, the target base station 3detects the preamble₂ from a next expected point corresponding to thesearch region at which the AI₁ is transmitted in step 52. At the step52, if the preamble₂ is detected, the AI₂ is transmitted in step 44 tothe mobile station 1 and the searching process is terminated. And a SFN,at which the AI₁ is transmitted, is transmitted to the base stationcontroller. If the preamble₁ or the preamble₂ are not detected, theabove-mentioned procedures are performed repeatedly.

The base station controller calculates a frame offset value when itreceives both of the CFN and SFN from the mobile station 1 and the basestation and produces a handover instruction. In case that the basestation controller receives a message of the failure to detect the AI₂from the mobile station 1, does not receive the SFN for a certainduration after receiving the CFN, does not receive the CFN apredetermined duration after receiving the SFN, or receives noinformation for a time of TGPRC from the target base station 3 and themobile station 1, the above-mentioned all steps are preformed again.

The above-mentioned steps of the method of the present invention can beimplemented as a program and can be stored in a computer readablerecording medium such as CD-ROM, RAM, ROM, floppy disk, hard disk andmagneto-optical disk.

The present invention, as mentioned above, transmits a preamble (or apilot), which is direct sequence spread to a new frequency for a shorttime by using an uplink compressed mode or a similar method to theuplink compressed mode before a mobile station completely disconnects acurrently established communication in an inter-frequency hard handoversituation described in FIG. 1. As a result, the present invention makesthe seamless inter-frequency hard handover possible by allowing thetarget base station to acquire the uplink synchronization before themobile station completely disconnects the currently establishedcommunication. Furthermore, in case that the base station controller 4does not know the frame offset, which is a difference between the SFN ofthe target base station and the CFN of the mobile station, the presentinvention can prevent a call disconnection by forcing the network toknow the frame offset by using an AI, which is transmitted through adownlink by using a new frequency from the target base station, justbefore performing the hard handover.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. A method for performing seamless inter-frequency hard handover in aradio communication system, comprising the steps of: a) a mobilestation, disconnecting a first uplink carrier frequency used forcommunication, transmitting a direct sequence spread preamble signalthrough a second uplink carrier frequency for a short time, andcontinuously performing the communication through the first uplinkcarrier frequency; b) a target base station, acquiring an uplinksynchronization of the mobile station by using the direct sequencespread preamble; and c) performing the hard handover by using the uplinksynchronization.
 2. The method as recited in claim 1, further comprisingthe steps of: d) the target base station, after acquiring the uplinksynchronization of the preamble, transmitting an AI as an acknowledgmentof the acquisition of the uplink synchronization by using a new downlinkfrequency; and e) the mobile station, detecting the AI.
 3. The method asrecited in claim 2, further comprising the step of f) re-transmittingthe preamble by increasing a transmission power if the mobile stationdoes not receive the AI from the target base station.
 4. The method asrecited in claim 2, further comprising the step of f) in case a basestation controller does not know a frame offset, which is a differencebetween a connection frame number (CFN) of the mobile station and asystem frame number (SFN) of the target base station, calculating theframe offset by using the SFN used for transmitting the AI and a CFNused for receiving the AI, which are transmitted from the target basestation and the mobile station, respectively, and transmitting thecalculated frame offset to the target base station.
 5. The method asrecited in claim 1, further comprising the step of d) in case a basestation controller knows a frame offset, the target base station,acquiring the uplink synchronization by using the preamble and reportingthe acquisition of the uplink synchronization only to the base stationcontroller.
 6. The method as recited in claim 5, wherein the step a)includes the steps of: a1) the base station controller, transmitting atransport channel frame number (TGCFN), at which the mobile stationstarts an uplink preamble transmission, a slot number (TGSN) andcompressed mode pattern parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 andTGPRC, to the mobile station after a time0 at which the mobile stationis communicating with a source base station through the first carrierfrequency and acquires the synchronization of a new downlink, the targetbase station has resources to support the mobile station but does notacquire the uplink synchronization and the base station controller knowsthe frame offset and a chip offset; and a2) the mobile station,transmitting the preamble during a transmission gap (TG) of acorresponding compressed frame through the second uplink carrierfrequency by using compressed mode information, the step b) includingthe steps of: b1) the base station controller, transmitting the TGCFN,the TGSN, the mode parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC,a scrambling code identification (SCID), the frame offset and the chipoffset to the target base station, thereby allowing the target basestation to search the preamble transmitted from the mobile station in apreset time; b2) the target base station, searching the preamble byusing the compressed mode information, the frame offset, the chip offsetand the SCID; and b3) the target base station, after achieving thepreamble searching, reporting the success of searching the preamble tothe base station controller by the target base station, and the step c)including the steps of: c1) the base station controller, instructing thesource base station and the mobile station to perform the handover andat the same time, transmitting orthogonal variable spreading factor(OVSF) information, which is used for establishing a new link, atransport channel frame number, of the mobile station, at which thehandover is performed, CFN and information for an uplink transmissionpower, which is used by the mobile station just after the handover isperformed; c2) the base station controller, instructing the target basestation to perform the handover and at the same time, transmitting theCFN at which the handover is performed; and c3) the mobile station andthe target base station, establishing communication there betweenthrough the new link at the CFN.
 7. The method as recited in claim 1,further comprising the step of d) in case a base station controllerknows a frame offset, the target base station, transmitting an AIthrough a new downlink of the second carrier frequency after acquiringthe uplink synchronization by using the preamble and reporting theacquisition of the uplink synchronization the base station controller.8. The method as recited in claim 7, wherein the step a) includes thesteps of: a1) the base station controller, transmitting a transportchannel frame number (TGCFN), at which the mobile station starts anuplink preamble transmission, a slot number (TGSN), compressed modepattern parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, andorthogonal variable spreading factor (OVSF) code information to be usedas a channel spread code of the AI through the new downlink to themobile station after a time0 at which the mobile station iscommunicating with a source base station through the first carrierfrequency and acquires the synchronization of a new downlink, the targetbase station has resources to support the mobile station but does notacquire the uplink synchronization and the base station controller knowsthe frame offset and a chip offset; and a2) the mobile station,transmitting the preamble during a transmission gap (TG) of acorresponding compressed frame through the second uplink carrierfrequency by using compressed mode information, the step b) includingthe steps of: b1) the base station controller, transmitting the TGCFN,the TGSN, the compressed mode parameters, TGL1, TGL2, TGD, TGPL1, TGPL2and TGPRC, a scrambling code identification (SCID), the frame offset andthe chip offset to the target base station, thereby allowing the targetbase station to search the preamble transmitted from the mobile stationis a preset time; b2) the target base station, searching the preamble byusing the compressed mode information, the frame offset, the chip offsetand the SCID; b3) the target base station, after achieving the preamblesearching, transmitting an AI spread to the OVSF through the newdownlink as a response of the success of searching the preamble; and b4)the target base station, after achieving the preamble searching,reporting the success of searching the preamble to the base stationcontroller, and the step c) including the steps of: c1) the base stationcontroller, instructing the source base station and the mobile stationto perform the handover and at the same time, transmitting a connectionframe number (CFN) of the mobile station, at which the handover isperformed, and information for an uplink transmission power, which isused by the mobile station just after the handover is performed; c2) thebase station controller, instructing the target station to performhandover and at the same time, transmitting the CFN at which thehandover is performed; and c3) the mobile station and the target basestation, establishing communication there between through a new link atthe CFN, wherein the OVSF code, which is used for transmitting the AI,is used for a communication channel of the new downlink.
 9. The methodas recited in claim 2, wherein, if the mobile station employs asingle-mode receiver, a last frame of the first carrier frequency forthe communication established between the mobile station and a sourcebase station and a first frame after the preamble is transmitted to thetarget base station are operated as compressed frames and, between thetwo frames, there is a guard time for a frequency conversion of themobile station.
 10. The method as recited in claim 4, wherein the stepa) includes the steps of: a1) for making the base station controllerknow the frame offset just before instructing the mobile station and thetarget base station to perform the handover, the base stationcontroller, transmitting a transport channel frame number (TGCFN), atwhich the mobile station starts an uplink preamble transmission, a slotnumber (TGSN), compressed mode pattern parameters, TGL1, TGL2, TGD,TGPL1, TGPL2 and TGPRC, and an orthogonal variable spreading factor(OVSF) code information to be used as a channel spread code of the AIthrough a new downlink to the mobile station after a time0, at which,the mobile station is communicating with a source base station throughthe first carrier frequency and acquires the synchronization of the newdownlink, the target base station has resources to support the mobilestation but does not acquire the uplink synchronization and the basestation controller knows a chip offset but does not know the frameoffset; and a2) the mobile station, transmitting the preamble during atransmission gap (TG) of a corresponding compressed frame through thesecond uplink carrier frequency by using compressed mode information,the step b) including the steps of: b1) the base station controller,transmitting TGCFN, the TGSN, the compressed mode parameters TGL1, TGL2,TGD, TGPL1, TGPL2 and TGPRC, a scrambling code identification (SCID) anda chip offset to the target base station, thereby allowing the targetbase station to search the preamble transmitted from the mobile stationin a preset time; b2) the target base station, searching the preamble ineach frame by using the compressed mode information, the chip offset andthe SCID from directly after the target base station receives a preamblesearch instruction from the base station controller; b3) the target basestation, after achieving the preamble searching, transmitting an AIspread by the OVSF through the new downlink as a response of the successof searching the preamble search to the mobile station; b4) the targetbase station, reporting an SFN of the target base station, whichtransmits the AI, to the base station controller; and b5) the mobilestation, reporting a CFN of the mobile station, at which the AI isdetected, to the target base station after succeeding to detect the AIand stopping transmitting the preamble, and the step c) including thesteps of: c1) the base station controller, instructing the source basestation and the mobile station to perform the handover and at the sametime, transmitting a connection frame number (CFN) of the mobilestation, at which the handover is performed, and information for anuplink transmission power, which is used by the mobile station justafter the handover is performed; c2) the base station controller,instructing the target station to perform the handover and, at the sametime, transmitting the CFN of the mobile station, at which the handoveris performed, and the frame offset to the target base station; and c3)the mobile station and the target base station, establishingcommunication there between through a new link at the CFN, wherein theOVSF code, which is used for transmitting the AI, is used for acommunication channel of the new downlink.
 11. A method for performing aseamless inter-frequency hard handover in a radio communication systemin case that a base station controller (or a radio network) dose notknow a frame offset, which is a difference between a connection framenumber (CFN) of a mobile station and a system frame number (SFN) of atarget base station, comprising the steps of: a) the mobile station,completely disconnecting a first uplink carrier frequency used forcommunication, transmitting a direct sequence spread preamble (or pilot)through a second uplink carrier frequency for a short time, andcontinuously performing the communication through the first uplinkcarrier frequency; b) the target base station, acquiring an uplinksynchronization of the mobile station by using the preamble beforeperforming the hard handover; c) the target base station, afteracquiring the uplink synchronization, transmitting a direct sequencespread AI as a response for the acquisition of the uplinksynchronization for a short time through a new downlink frequency; d)the mobile station, detecting the AI; e) the base station controller,calculating a frame offset by using the SFN, which is used fortransmitting the AI and the CFN, which is used for receiving the AI, andtransmitting the calculated frame offset to the target base station; andf) the base station controller, instructing the mobile station and thetarget base station to perform the handover.
 12. The method as recitedin claim 11, wherein the method uses an orthogonal variable spreadingfactor (OVSF) code, which is used in a communication channel of thedownlink when hard handover is performed through a new link, as achannel spreading code of the AI in case that the target base stationtransmits the AI as the response for the acquisition of the uplinksynchronization through the new downlink frequency.
 13. The method asrecited in claim 12, wherein the target base station transmits the AI atleast more than one time to increase the probability of detecting the AIof the mobile station.
 14. The method as recited in claim 11, whereinthe mobile station uses an uplink compressed mode pattern for theinter-frequency hard handover.
 15. The method as recited in claim 14,wherein the mobile station uses a scrambling code operating in a normalmode when transmitting the preamble while using the uplink compressedmode pattern for the inter-frequency hard handover.
 16. The method asrecited in claim 11, wherein the occurrence of false handoverinstruction by the base station controller is minimized by classifyingthe AI to a first AI (AI₁) and a second AI (AI₂), and the preamble to afirst preamble (preamble₁) and a second preamble (preamble₂) fordecreasing the false detection probability of the target base stationand the mobile station in case the base station controller does not knowthe frame offset.
 17. The method as recited in claim 16, wherein the AI₁and AI₂ are distinguished with each other according to a binaryorthogonal transform method using an orthogonal code.
 18. The method asrecited in claim 16, wherein the preamble₁ and the preamble₂ use anidentical scrambling code, and are distinguished with each otheraccording to a binary orthogonal transform method using a orthogonalcode.
 19. The method as recited in claim 11, wherein the mobile stationperforms a coherent demodulation to detect the AI by using a commonpilot channel (CPICH) transmitted through the frequency through whichthe AI is transmitted.
 20. The method as recited in claim 11, whereinthe target base station stores signals received during a transmissiongap (TG) of a compressed mode, at which the mobile station transmits thepreamble, and then searches the preamble by using signals stored in atime, which the mobile station does not transmit the preamble.
 21. Themethod as recited in claim 16, wherein the step c) includes the stepsof: c1) the target base station, transmitting the first AI (AI₁) themobile station through the new downlink frequency as a response for thesuccess of searching the first preamble; c2) the target base station,detecting the second preamble (preamble₂) in a next compressed frameafter transmitting the first AI (AI₁); c3) the target base station,transmitting the second AI (AI₂) to the mobile station when detectingthe second preamble (preamble₂), reporting a success of acquiring theuplink synchronization to the base station controller and at the sametime of noticing a SFN at which the second AI (Al₂) is transmitted; andc4) the target base station, detecting the first preamble again when itfailed to detect the second preamble (preamble₂), the step d) includingthe steps of: d1) the mobile station, transmitting the second preamble(preamble₂) in the next compressed frame with the same power as usedbefore when succeeding to detect the first AI (AI₁) and transmitting thefirst preamble (preamble₁) again when failing to detect the first AI(AI₁); d2) the mobile station, reporting the detection of the second AI(AI₂) to the base station controller in case that the mobile station issuccess to detect the second AI (AI₂) after transmitting the secondpreamble (preamble₂) and, at the same time, transmitting a CFN, at whichthe second AI (AI₂) is detected to the base station controller; and d3)the mobile station, reporting the failure of detecting the second AI(AI₂) to the base station controller in case that the mobile stationfails to detect the second AI (AI₂), after transmitting the secondpreamble (preamble₂) the step e) including the steps of: e1) the basestation controller, calculating the frame offset when it received theCFN and the SFN from the mobile station and the target base station,respectively; e2) the base station controller, instructing a home basestation and the mobile station to perform the handover and, at the sametime, transmitting the CFN of the mobile station, which performs thehandover, and information for an uplink transmission power to be used bythe mobile station directly after performing the handover; and e3) thebase station controller, instructing the target base station to performthe handover and, at the same time, transmitting the frame offset andthe CFN of the mobile station, which performs the handover, and the stepf) including the steps of: f1) establishing a new communication linkbetween the mobile station and the target base station at the CFN,wherein a downlink of the new communication link uses an orthogonalvariable spreading factor (OVSF) code, which is used in transmitting theAI; and f2) repeating the steps c1) to e3) in case that the base stationcontroller receives a message of failing to detect the AI₂ from themobile station, does not receive the SFN for a certain time afterreceiving the CFN, does not receive the CFN for a preset time afterreceiving the SFN, or does not receive any information from the mobilestation and the target base station until a predetermined time isexpired.
 22. A computer readable record medium for storing instructionsfor executing seamless inter-frequency handover, comprising thefunctions of: a) a mobile station, disconnecting a first uplink carrierfrequency used for communication, transmitting a direct sequence spreadpreamble signal through a second uplink carrier frequency for a shorttime, and continuously performing the communication through the firstuplink carrier frequency; b) a target base station, acquiring an uplinksynchronization of the mobile station by using the direct sequencespread preamble; and c) performing the hard handover by using the uplinksynchronization.
 23. The computer readable record medium as recited inclaim 22, further comprising the functions of: d) the target basestation, after acquiring the uplink synchronization of the preamble,transmitting an AI as an acknowledgment of the acquisition of the uplinksynchronization by using a new downlink frequency; and e) the mobilestation, detecting the AI.
 24. The computer readable record medium asrecited in claim 23, further comprising the function of f)re-transmitting the preamble by increasing a transmission power if themobile station does not receive the AI from the target base station. 25.The computer readable record medium as recited in claim 22, furthercomprising the function of d) forcing the base station controller toknow a frame offset just before the base station controller (or a radionetwork) instructs the mobile station and the target base station toperform the handover in case that the base station controller does notknow the frame offset, which is a difference between a connection framenumber (CFN) of the mobile station and the a system frame number (SFN)of the target base station, thereby preventing an additional calldisconnection.
 26. The computer readable record medium as recited inclaim 22, further comprising the function of d) in case a base stationcontroller knows a frame offset, the target base station, acquiring theuplink synchronization by using the preamble and reporting theacquisition of the uplink synchronization only to the base stationcontroller.
 27. The computer readable record medium as recited in claim22, further comprising the function of d) in case a base stationcontroller knows a frame offset, the target base station, transmittingan AI through a new downlink of the second carrier frequency afteracquiring the uplink synchronization by using the preamble and reportingthe acquisition of the uplink synchronization the base stationcontroller.